Before anyone asks, I do not have the regular SST, so I can't tell you how much better SST Extreme is. In case you're wondering, Extreme version differs from standard b/c it has finer silver flakes and is cryo'd. I tried the SST Extreme in 4 stages in order to isolate which stage benefits the most.
Stage I - Power
Power plugs ends, male IEC ends, fuse ends were done first. Of all the stages, the "power" stage was the only one that had a negative side effect for first 20 minutes or so, which consisted of a silvery sheen and an unnatural sparkle to high frequencies. But after about 10-20 minutes, this tendency completely went away, leaving more clarity, unfettered dynamics, and more immediacy to the sound. Dynamics, especially in bass, became more effortless, contributing to a slightly more forward sound, both in soundstaging and impact. There wasn't necessarily any change in tonality, treble/bass amount or extension at this point, though.
Stage II - Tube Pins
Very similar changes to stage I. No overt change in frequency response or tonality, but one more veil has been lifted. Contrary to what others have said, I didn't perceive a significant extension in extreme treble, but I did notice mid-treble to mid-bass area gaining meaningful amount of resolution and clarity. Not as much gain in dynamics or bass as stage I, but that may be b/c I did the stage I first. Soundstage/image became a tiny bit more forward once again.
Stage III - speaker cables, amp/speaker terminals
This is the stage that gives you the feeling volume is louder, which a lot of SST users report. I didn't perceive loud volume with stage I or II, but III definitely made things sound louder. I had to turn down volume by one step in Foobar to go back to the usual volume level. There's definite improvement in bass definition and slam, to the point some room bass nodes became less problematic. A little more of clarity and resolution across the ranges, and treble sounded cleaner and a tad more extended, but not a huge amount. Once again, just a teeny bit more forwardness/immediacy.
Stage IV - interconnects
This is the treble extender. More than any stage, treating interconnects gives you very noticeable treble extension. It's the kind of extension that makes the sound Less "bright" b/c the extreme top treble actually makes the whole sound more silky, coherent, and less peaky in low-treble/upper midrange. The increasing forwardness I've described through stage I-III takes a step back, especially in midrange. Before stage IV, the vocals perhaps became a tiny bit more upfront and forward than natural, but after stage IV, high treble air suffuses the soundstage while the slightly forward midrange now takes a step backward to perfectly balance out the ranges.
I'm actually a heavy afacionado of that thick, forward, dense midrange presence, but I rather liked the more balanced effect of treating the interconnects.
The other pleasant surprise was how soundstage depth improved while soundstage volume expanded in all directions. At the same time, individual voices/instruments focused more tightly into denser sound sources without becoming unnatural "pinpoint source."
Another bonus was the elimination of certain slight chaulkiness and teeny peakiness I assumed was inevitable due to use of metal(any and all metal) and dielectrics. Sound gained tremendous coherence and fluidity that I thought I had to spend $$$ to achieve (Cyberlights or Silversmith Palladiums, for example). I wonder what would happen if one compared a good RCA +SST Extreme vs. Eichman or Nextgens
Overall a worthy tweak, well worth the $150 MSRP. I thought SST Extreme was overpriced in the past, but given the magnitude of honest improvement it brought, I no longer think so. By "honest," I mean no subtle manipulations of frequency response, emphasis of certain parts to achieve an artificial "improvement" like many tweaks out there.
BTW, Please realize I had to unambiguously state the differences clearly in order to communicate what I heard, but this usually results in readers overestimating the magnitude of improvements described. I'm talking about meaningful, real improvements, but not "OMG, my system transformed!" variety.
As with all tweaks, it may be system-dependent and YMMV, etc.